Filter with stiffening ribs

ABSTRACT

Annular wire mesh filters for airbag assemblies for passenger vehicles are provided with a series of ribs extending parallel with the axis of the annulus and along the outer wall. The ribs provide added strength to the filter. Also provided is a method of manufacturing the compressed mesh filter with a uniform density, and a molding tool (mold, mandrel, and plunger) for making the compressed mesh articles of this invention.

This application is based on provisional application number 60/72,325filed Jan. 23, 1998. This application claims the benefit of priorforeign application GB 9827576, filed Dec. 15, 1998.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

This invention relates to filters, and especially filter elements usefulfor filtering hot gases used in the deployment of passenger airbags, andto methods for making and using such filters, and to airbags andvehicles containing the same.

2. The State of the Art

Relatively recent concerns with passenger safety in land vehicles hasled to the development of “airbag” technology, a passive restraint andprotection system comprising a bag or pillow-like bladder that isinflated in an extremely short period of time using compressed orchemically-generated gas to fill the bag. The inflated bag is disposedor deployed between the front or side of the passenger and an interiorportion of the vehicle's passenger compartment.

The first generation of pyrotechnic airbag vehicle occupant restraintsystems used azide compositions (typically sodium azide, NaN₃, mixed aheavy metal oxide) to generate the gas used to inflate the airbag. Theseexplosive compositions generate a gas at over 1,000° F. during theinitial phase of the gas generation reaction. A large amount ofcondensable and molten and/or solid particulate matter is generatedconcurrently with the gas. Much of this matter is not only extremely hotbut also of a caustic composition, and the particulate matter,travelling a high velocity, is potentially dangerous to the integrity ofthe bag and the occupant to be protected thereby. Some airbag designsincluded large vent holes in the bags for venting the gas into thepassenger compartment, and so the gas used to inflate these bags must befiltered to prevent the particulates from entering the passengercompartment with the vented gas. In these designs, all of the gasgenerated escapes the reaction chamber and is propelled towards theairbag, so that the gases and any particulates would undoubtedly impingeat least on the bag itself if no filter were present. If no measures aretaken to ameliorate the degradative effects of this mixed phase reactionmixture, the gases and/or particulates would penetrate the bag, likelycausing its failure and, in the most serious situations, causinginjuries to the passenger.

Various measures were taken to reduce the degradative effects of thegas, some of which are discussed in U.S. Pat. Nos. 4,902,036 and5,318,323, both of which are incorporated herein by reference. Onetechnique for reducing the degradative effects was the use ofsacrificial layers to slow down the particulate material, and the use ofstatic centrifugal or impingement particle separation techniques. Theart also resorted to using denser and/or longer filter devices. Withboth of these approaches, there is a design trade-off between filteringthe gases and providing a pressure drop small enough to avoidinterfering with the rate at which the airbag inflates. There are othertrade-offs, as particulate deflection devices are typically expensivemachined parts which are fabricated from heavy steel plate (because theyare not amenable to fabrication by stamping, their cost of manufacturingis increased). In general, the airbag designers had to contend withremoving condensed solid poisonous products (usually unreacted sodiumazide and sodium oxides produced in the reaction), cooling the gasesbefore they inflated the cushions, and providing a homogeneous anduniformly distributed gas flow generated from an explosive source.

Many filtering devices used today comprise layers of metal screens ofvarious mesh sizes and one or more layers of a non-combustible fibrousmaterial packed between the screens. The efficiency of this type offilter is dependent upon how tightly the material is packed; a tighterpacking leads to more efficient filtering but also to a higher pressuredrop. According to the above-referenced '323 patent, there is also aproblem with quality control in the mass fabrication of such screen-matcomposites with respect to providing a uniform pressure drop across anygiven filter made.

Yet another problem in designing airbag filter devices is that as thefilter becomes clogged, the pressure drop across the filter increases.Accordingly, the mechanical stresses on the filter are increased, andthe gas and particulates move through the filter at a higher velocity,necessitating an improved filter strength and toughness to withstand thehigher flow rate through, pressure drop across, and particulate velocityinto the filter.

Besides the aforementioned patents, typical filters for airbags are madefrom a compressed wire mesh or steel wool, such as described in U.S.Pat. No. 3,985,076 (metallic mesh), EP 674,582 (sintered metallic fiberstructure), U.S. Pat. No. 4,017,100 (multilayer structure of glassfibers, steel wool, and screens and perforated plates), DE 2,350,102(glass wool), GB 2,046,125 (metal spheres partially sintered together toform a rigid, porous body), U.S. Pat. No. 5,204,068 (metal fiber feltcomprising coated fibers, such as nickel, coated with siliconcompounds), WO 94/14608 (metal wire mesh to which a non-woven web ofmetal fibers is bonded by sintering), and others, the disclosures ofwhich are all incorporated herein by reference. The gas generatingcomposition, often an azide (azoimide) composition with copper,generates hot gases and particles of copper slag. The desire of thedesigner is to filter the copper slag particles so that the molten metaldroplets do not impinge the airbag. The final filter design became atrade-off between (i) having a sufficiently high density of filtermaterial to catch the slag particles, (ii) providing sufficient mass inthe filter to cool the filtered slag particles before they melt throughthe mesh or wool elements of the filter or fragment into smallerdroplets that might do the same, and (iii) the total density and weightconstraints of the filter. That is, if the filter is made of very finewire mesh or wool to assure catching all of the molten slag particles,then the mesh or wool fibers will have insufficient mass to cool theimpinged slag particle to a solid, and so the molten slag particle meltsthrough the mesh or wool and/or it fragments into smaller particles thatmay eventually pass through the filter and impinge the airbag.

The new generation of gas generators employ cleaner and less toxicnon-azide gas-generating compositions (e.g., as described in U.S. Pat.No. 5,525,170, disclosure of which is incorporated herein by reference)that provide relatively more gas than the azide-based compositions.While the need to filter the gas generated is thus less of a concern,federal government standards exist setting limits on the allowableamounts of soluble and insoluble particulates in the gas generated, andso there is still a need to filter the gas. The need to cool the gasgenerated is still a necessary step in the deployment of the airbag.Moreover, these newer generation gas generators still yield asignificant explosive force against which the filter element must bestabilized.

SUMMARY AND OBJECTS OF THE INVENTION

One object of this invention to provide a relatively inexpensive and yettough filter for airbags and similar inflatable passive safety devices.

Another object of this invention is to provide a simple airbag filterthat has reinforcement to better withstand the explosive force of thegas generation.

Still another object of this invention is to provide a light-weight,inexpensive, and effective filter device for airbags.

These and other objects of the invention are achieved in one aspect by awire mesh filter deformed to create a plurality of ribs around thecircumference of the filter. The filter can be deformed by being pressedinto a mold. The circumference of ribs provides an improved hoopstrength for the filter.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A-1C depicts idealized front, side, and rear views of a moldingtool used to fabricate the novel filters of this invention.

FIGS. 2A-2D depict, respectively, two perspectives, an end view, and aside view, of filters having ribs made according to the presentinvention.

FIGS. 3A and 3B depict the plunger and the combination female mold andmandrel, respectively, for this invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In brief, wire of a particularly chosen type and diameter is knittedinto a knit mesh tube having a particular width and density for thefiltering application desired. A piece of the mesh tube is cut to aparticular weight that is a function of the weight and filteringrequirements of the environment and fluid to be filtered. The mesh tubeis then pressed into the desired annular shape of a filter using afemale mold, a mandrel, and a plunger or press to produce a filterhaving the desired physical dimensions, weight, and density. The annularfilter is then further shaped in a mold to deform the outer annularcircumference into a series of ribs.

Shown in FIG. 1 (left to right) are a top, side, and bottom view of themold 101 used to form the ribs on the mesh filter. The mold ispreferably made of a hard tool steel. The mold has an internal cavity103 defined by an inner annular circumference. The tool shown in FIG. 2is used to make a filter having ribs as shown in FIGS. 2A and 2D, inwhich the ribs extend along only a portion of the axial length of thefilter. Returning to FIG. 1, to make the filter shown in FIGS. 2A and2D, the tool preferably has two inner annular circumferences 105 and107. One of the annular circumferences includes a plurality of groovesor channels 109 spaced equally about the circumference and extendingparallel with the axis of the mold (orthogonal to the circumference). Asthe mesh filter is forced into the tool by the plunger, the channelsdeform the outer perimeter of the filter so that the mesh occupies thewhole of the internal cavity of the molding tool. Thereafter the filterwith the ribs is removed from the molding tool.

Turning to FIGS. 2A-D, FIG. 2A shows an idealized perspective view ofthe filter made with the tool shown in FIG. 1. The molding tool deformsthe outer perimeter of the filter so that the mesh occupies the channelsin the molding tool. When demolded, the outer perimeter of the meshfilter will have a plurality of ribs 201 spaced about the outerperimeter. Because the tool of FIG. 1 has two inner annularcircumferences (i.e., having different circumferences or onecircumference having channels and the other devoid of channels), theribs of filter of FIG. 2A extend only partially along the axial lengthof the filter. FIG. 2D depicts a side view of the filter of FIG. 2A. Inthe alternative, the molding tool can have a single inner cavity andinner annular circumference with channels extending along the entireinner axial length of the molding tool. When such a tool is used, theresulting filter has a geometry as shown in FIG. 2B, in which the ribsextend along essentially the entire axial length of the filter element.FIG. 2C is an idealized end view of the filter of FIG. 2A or 2B whenviewed from the end of the filter to which the ribs extend.

The preferred filter elements in the present invention comprise a wiremesh compressed into a desired geometry, preferably annular, andpreferably circular or elliptical (oval). The mesh is preferablyproduced by a conventional wire knitting machine (such as anycommercially available wire mesh knitter, as are available from, forexample, Tritech International, England); examples of wire meshes andknits used as scals and support mats in high temperature applicationscan be found in U.S. Pat. Nos. 4,683,010 and 5,449,500, the disclosuresof which are incorporated herein by reference. The wire knitting machinetypically produces a pliable mesh sleeve.

The wire used to make the mesh can be of various compositions, andpreferably is selected from stainless steels, including austenitic andnickel alloys, such as, but not limited to, 304, 309, and 310 grades ofstainless steel. The composition of the wire is chosen to be chemicallycompatible (to the extent possible) with the environment in which thefilter is disposed and with the fluid (or mixed phases) being filtered.Accordingly, other metals, and even polymeric fibers, can be used,depending upon the environment and the properties of the materials beingfiltered, and to the extent that such can be formed into a filter havingribs spaced along its outer circumference.

The wire for the mesh used for fabricating airbag filters preferablyranges from about 0.03 in. dia. to about 0.002 in. dia. (from about 0.75mm to about 0.05 mm in diameter, or from about 21 to about 47 gauge(Brit. Std.); although larger and/or smaller wire can be used). Ifmultiple meshes are used together in a single filter, it is preferredthat the largest diameter wire be used for the innermost filter zone(s)and that smaller gauge wires be used for the outermost filter zone(s),and that the wire size decrease in the radially outward direction. Asmentioned above, the explosive charge releases particulates of moltenmetal (slag) that impinge the filter. The use of a thicker mesh wireand/or a more dense radially interior portion tends to ameliorate thedeterioration of the filter due to the corrosive mixture. For example,near the center of the filter where the charge explodes and slag isformed that impinges the filter; a thicker wire (a) has a higherstrength than a thinner wire to better absorb the explosive force and(b) has a greater effective heat capacity that can tolerate a largerand/or hotter slag particle better than a relatively thinner wire (e.g.,before a molten slag particle burns through the wire).

The wire used for the entire filter, or any of the individual parts orsections of the filter, can be round or flat in cross-section. The wireused also can be a combination of two or more different geometriesand/or compositions of wire. Different types, diameters, and/orgeometries of wire can be knit into a single mesh to provide a meshhaving a uniform composition of different wires or a composition ofwires that changes along the length of the mesh tube. Further,additional strength can be obtained by heat treating; e.g., annealingthe filter in an oxygen-containing atmosphere (such as ambient); such anannealing process is described in the aforementioned U.S. Pat. No.5,449,500 (the disclosure of which is incorporated herein by reference).The same wire can be used for two different sections of the filter andcompressed or compacted to provide a different density in each section.Likewise, different wires (regarding geometry and/or composition) can beused to produce different filter sections each having the same density.Besides a wire mesh and steel wool, one or more sections of the filtercan include other types of wire filter media (such as those commerciallyavailable from Memtec, Ltd., Australia). Such media may also comprise acompacted and/or annealed wire mesh, and if obtained separately, can befabricated into a desired shape (e.g., a strip cut and welded into acircular loop) before being integrated with the compressed mesh of thepresent invention.

The density of the filter is typically specified by the designer of theentire airbag assembly. Knowing the volume of the filter (also a designconstraint based, for example, on the steering wheel size andconfiguration), and the specific density of the wire (stainless steeltypically has a density of about 0.29 lb./in.³), the density of thefilter can be determined. Thus, for any particular zone of the filterhaving a specified density, the weight of mesh required to fit into thatfilter zone volume can be calculated from the density.

It is preferred that the final filter article be made in a series ofcompressions starting with the knit wire tube. As has been noted, thedensity of the final filter is a design parameter of the air bagassembly. When it is desired to provide a filter having an essentiallyuniform density, it is preferred that the filter be formed in a seriesof compressions. In the first compression, the desired amount of knitwire tube is pressed into an annulus using a cylindrical female moldwith a mandrel (to provide the outer and inner diameters of the annulus)and a plunger in the geometry of a sleeve to force the knit tube intothe space between the mandrel and the female mold. In the mold, thisintermediate article can be defined with reference to base end at thebottom of the mold, and a work end contacted by the plunger. Typically,for example, 14 inches of knit wire tube is compressed into a 3½ to 4inch annulus (measured along the axis of the annulus).

Thereafter, the intermediate annulus is placed into a mold of the typeas shown in FIG. 1, again comprising a female mold and a mandrel, and asleeve plunger is used to press the annulus so that it conforms to themandrel and the female mold circumference having the grooves,effectively causing the wire mesh to flow. To diminish the unevencompression, and thus uneven density, that is likely to occur, the“work” end of the intermediate annulus is placed first into the mold sothat it becomes the “base” end in the next operation; that is, theintermediate article is flipped-over so that it is compressed from theopposite end than it was originally. In this next step, the 3½ to 4 inchintermediate annulus is compressed into an annulus about one to twoinches in length (along the axis of the annulus). To facilitate thissecond compression molding step, the dimensions of the mold used in thefirst compression molding step should provide an annulus having a largerI.D. and a smaller O.D. than the final article, so that the intermediateannulus easily fits over the mandrel and into the mold used to make thefinal article. Presses delivering 70 to 80 tons of pressure, andpossibly up to 100 tons, are required to deform the mesh and producethese articles.

The apparatus used for the molding is shown also in FIGS. 3A and 3B. Inparticular, FIG. 3A depict a mandrel for use with this invention. Themandrel has a disk shaped base 301 to which a mandrel 303 is attachedorthogonally in the center. The size of the base should be such that itcan be fit securely into the bottom of the female mold, as shown in FIG.3B (in which the grooves are not shown). The knit mesh tube or theintermediate annulus is positioned over the mandrel that is positionedin the female mold cavity, and the sleeve plunger 305 is pressed intothe mold. The plunger has an outer diameter 307 that corresponds withthe inner diameter of the female mold, and an inner diameter 309corresponding with the outer diameter 311 of the mandrel. As in thepresent invention where it is desired to provide ribs on the outside ofthe compressed mesh article and there are grooves on the inner wall ofthe female mold, the plunger has ribs corresponding to the grooves onthe inner wall so that a good seal is made with the mesh beingcompressed and to assure that the mesh flows into the mold geometrywhere desired (that is, otherwise the mesh will flow around the plungerand the end of the compressed article will not be flat).

In certain embodiments it may be desirable to have a relatively longcompressed mesh article that is too long for the mold (or a mold of thedesired length would be too expensive). In such cases, multiple annularcompressed mesh articles can be joined end to end, preferably by meansof a joint. The preferred joint is a tongue-and-groove configuration,wherein an intermediate portion of the article would have a tongue inone end and a groove in the other. This can be accomplished by alteringthe configuration of the base of the mandrel and the working end of theplunger. In particular, a circular groove or ridge can be formed in themandrel base so that when the wire mesh is forced there-against by theplunger, a tongue or groove (respectively) will be formed in thecorresponding abuting end of the compressed mesh article. The workingend of the plunger is modified accordingly to have the oppositeconfiguration of a ridge or groove, thereby forming a groove or tonguein the opposite end of the compressed mesh article. Thus, compressedmesh articles formed this way have a groove on one end and a tongue onthe other, and so can be joined end to end to provide a longer (axially)article.

When the compressed mesh articles of this invention are used in airbags,the airbag manufacturer provides a can into which the mesh filter isinserted, and into the annulus the explosive charge is loaded (with aprimer) and the can is sealed. The can includes a number of vent holesthrough which the gas generated escapes, and the holes are usuallysealed with paper as a barrier (e.g., against water). Prior to thisinvention, the manufacturer would have to insert a locator plenum intothe can to locate the position of the filter, and then around the filtera welded, perforated tube would be inserted to provide increased hoopstrength, the plenum is used not only to locate the filter but also toassure that the filter did not touch the walls of the can and compromisethe seals of the holes in the can. By virtue of this invention, both thelocator plenum and the perforated tube can be eliminated from themanufacturing process, providing a significant cost savings and ease ofmanufacturing. As mentioned above, the ribs provide improved hoopstrength sufficient to eliminate the perforated tube. The filter asshown in FIG. 2A can be used to eliminate the locator plenum because theribs act as registrations to center the filter in the can. Additionally,because the ribs extend only partially along the length of the filterand the portion without the ribs has a smaller diameter, the smallerdiameter portion can be adjacent the portion of the can having the holesand still remain sufficiently far away so that there is a minimal chanceof the seals covering the vent holes being compromised because ofcontact with the filter.

The novel articles of this invention are also suitable for making meshsubstitutes for mechanical attenuation of movement, especially forabsorbing energy, restricting movement, or providing a flexing motion.These articles are thus useful as substitutes for rubber bushings andflextubes (flexible cylindrical or annular devices for connectingconduits).

The foregoing description is meant to be illustrative and not limiting.Various changes, modifications, and additions may become apparent to theskilled artisan upon a perusal of this specification, and such are meantto be within the scope and spirit of the invention as defined by theclaims.

1. A wire mesh filter for use in an airbag inflator assembly including agas generator and an airbag; the wire mesh filter including a knittedwire mesh having an annular geometry defined by an axis with two endsand an outer cylindrical wall and a plurality of at least four ribsdirected along the axis and extending at least partially along the outercylindrical wall, the ribs comprising the mesh filter; wherein gasesthat are explosively generated by the gas generator pass into andthrough the wire mesh filter in order to inflate the airbag.
 2. Theimproved filter of claim 1, wherein the ribs extend fully along theouter wall between the two ends.
 3. The improved filter of claim 1wherein the wire comprises 304 or 309 stainless steel.
 4. An improvedairbag assembly comprising a gas generator, an inflatable bag, and afilter through which gas explosively generated passes into and inflatesthe bag, wherein the improvement comprises a knitted wire mesh filterhaving an annular geometry defining an axis and an outer circumferentialwall, said outer circumferential wall being deformed into ribs parallelwith said axis, and said mesh filter having an annular geometry definedby an axis with two ends and an outer cylindrical circumferential wall,and a plurality of at least four ribs directed along said axis andextending along the outer cylindrical wall, the ribs comprising saidmesh filter.
 5. A passanger passenger vehicle having an airbag, whereinthe improvement comprises an airbag assembly comprising a gas generator,an inflatable bag, and a filter through which gas explosively generatedpasses into and inflates the bag, said filter being a knitted wire meshfilter having an annular geometry defining an axis and an outercircumferential wall, said outer circumferential wall being deformedinto ribs parallel with said axis, and said mesh filter having anannular geometry defined by an axis with two ends and an outercylindrical circumferential wall, and a plurality of at least four ribsdirected along said axis and extending along the outer cylindrical wall,the ribs comprising said mesh filter.
 6. The improved filter of claim 4,wherein said ribs extend only partially along the length of said axis.7. The improved filter of claim 1, wherein the distance between adjacentribs along said outer cylindrical wall approximates the width of eachsuch rib.
 8. The improved airbag assembly of claim 4, wherein thedistance between adjacent ribs along said outer cylindrical wallapproximates the width of each such rib.
 9. The improved airbag assemblyof claim 4, wherein said ribs extend only partially along the length ofsaid axis.
 10. The improved airbag assembly of claim 4, wherein thedistance between adjacent ribs along said outer cylindrical wallapproximates the width of each such rib.
 11. The improved airbagassembly of claim 9, wherein the distance between adjacent ribs alongsaid outer cylindrical wall approximates the width of each such rib. 12.A passenger vehicle having an airbag, wherein the improvement comprisesan airbag assembly comprising a gas generator, an inflatable bag, and afilter through which gas explosively generated passes into and inflatesthe bag, said filter being a knitted wire mesh filter having an annulargeometry defining an axis and an outer circumferential wall, said outercircumferential wall being deformed into ribs parallel with said axis,and said mesh filter having an annular geometry defined by an axis withtwo ends and an outer cylindrical circumferential wall, and a pluralityof at least three ribs directed along said axis and extending onlypartly along the outer cylindrical wall, the ribs comprising said meshfilter.